A novel quantitative phase analysis (QPA) based on the Rietveld profile refinement of neutron diffraction patterns and a newly developed quantitative texture analysis was examined for the determination of the weight fraction of retained austenite in TRIP steels after cold rolling or tensile deformation. Textures of both the bcc phase of ferrite and bainite and the fcc phase of retained austenite were successfully determined separately by means of neutron diffraction even in samples containing only a small percentage of austenite. The as-received TRIP steel displayed a typical rolling texture of both the bcc and fcc phases. The preferred orientation factors (POFs) extracted from the recalculated orientation distribution functions (ODFs) were applied to the QPA. Cold rolling or simple tension gave rise to the development of deformation textures in the bcc phases. However, the plastic deformation led to a weakening of the fcc texture. The transformation behavior of retained austenite during cold rolling and simple tension was analyzed by using the parameter of the effective strain.